FSK (frequency shift keying) is one of many modulation techniques widely used for digital data transmission. Often MODEMS are used to connect computers or other digital equipment over a transmission medium, such as a telephone loop, coaxial cable, fiber optics, electromagnetic waves etc. Voiceband FSK is used over telephone loops to transmit data between customers' premises equipment (CPE) and a switching office for local area signaling services. In this transmission, FSK signals are sent on mark and space tones whose frequencies are, for example, 1200 and 2200 Hz respectively and therefore lie in the voiceband. The FSK signals can be sent over a telephone loop while a customer's terminal, e.g. a telephone set, is either off-hook or on-hook. One popular use of FSK signals in the telephone environment is to display the telephone number of an incoming call on the customer's telephone set. Numerous other uses have been devised and can be conceived in the future for this data transmission between CPE and switching office.
A digital FSK demodulator using a quadrature phase detector is described in U.S. Pat. No. 5,155,446 (Eberle et al), issued on Oct. 13, 1992. The demodulator of this patent comprises a highpass filter, a quadrature phase detector, and a lowpass filter. The demodulated output of the lowpass filter is fed to a UART (asynchronous receiver) for data recovery. This demodulator uses mainly a collection of shift registers for the filters and one multiplier for the quadrature phase detector. It is therefore very easy and economical to manufacture in a small IC chip. Generally speaking, the sampling rate is four times the average of the signaling tones. In the embodiment discussed in the patent, the mean value of the two signaling tones is 1700 Hz; therefore the sampling rate of the modulated signal is 6800 Hz. Assuming that the amplitude of each received tone is unity (an unrealistic assumption in practice), the sampling rate is set at 6800 Hz, the DC gain of the lowpass filter is 0 dB, and the lowpass filter completely eliminates the harmonic component in the quadrature demodulator output (the last also being unrealistic), the output of the quadrature demodulator would be as shown in the table below.
______________________________________ Signaling Corresponding Frequency Normalized Radian Quadrature Output (Hz) Frequency .omega. (rad/s) After Lowpass Filtering ______________________________________ 1200 1.108797 0.2228692 2200 2.032795 -0.2228692 ______________________________________
The positive value at the output of the quadrature demodulator (after lowpass filtering) corresponds to the mark frequency (1200 Hz), while the negative value corresponds to the space frequency (2200 Hz). However, because certain assumptions are unrealistic, it is necessary to consider the effect of non-ideal conditions. The levels of the received signaling tones are not equal due to many factors, a few of which are mentioned below.
MODEM Transmission PA1 The MODEM may not necessarily transmit the tones at equal level. PA1 Loop PA1 The loop attenuates the space tone more than the mark tone. PA1 External Highpass Filtering PA1 Provided to attenuate 60 Hz noise, this filter attenuates the mark tone more than the space tone. PA1 Sinc.sup.2 Decimation Filter PA1 This filter has a lowpass characteristic attenuating the space tone more than the mark tone. PA1 Digital Highpass Filtering PA1 Like its external counterpart, this filter attenuates the mark frequency more than the space frequency.
As a result of the inequality in the amplitude of the received signaling tones, the quadrature outputs generated by each signaling tone no longer sum to zero, thereby creating an offset in the demodulator output. Since the zero crossings of the demodulator output determine the timing recovery in the asynchronous receiver, it is imperative that this offset be eliminated.